Engine Burn Question

[NickyUsa]

Say I have a spacecraft with eight engines, orbiting Kerbin. Would it be more efficient to burn all 8 engines? or just as few as possible?

[Vanamonde]

An engine that is not firing is dead weight, holding the ship back and reducing fuel efficiency. Either don't put them on there in the first place, or fire them all every time.

[theend3r]

If they all have the same Isp then fire all at once, if not then it's a bit more complicated.

[palioxis1248]

Yes. All engines should be firing at once, or else you have way too many engines and should throw the non-firing ones away. That is, of course, unless your engines are not meant for this part of the launch.

Thrust is another issue however. You generally don't want too much thrust while you're in thick atmosphere, because that wastes fuel due to drag.

[capi3101]

Smoke 'em if you've got 'em. Next time, consider dropping a few of 'em (especially if you're going interplanetary).

[Tank Buddy]

One engine is always more efficient than multiple engines when building a craft. So if you don't need the extra thrust, don't put them on. If you already have them on your ship, burning more engines does not decrease your efficiency because you are changing velocity (DV) faster, and with more thrust, and consuming fuel at the same rate per engine. You essentially are just changing velocity faster, where the only loss is the extra weight.

[Kasuha]

More engines means more TWR (thrust to weight ratio). That means you'll need less time to accelerate by certain amount of m/s.

Assuming these engines have all the same ISP (impulse per fuel spent), you should use them at once. If they don't, it might be better to use only engines with highest ISP unless you need to accelerate really fast.

[Claw]

Well, I would add that an engine that isn't firing isn't simply dead weight or wasted. If you have small maneuvers you want to perform then sometimes reducing the number of engines firing can help keep you from overshooting your target, thereby reducing the need to correct. Also, if you're carrying an inefficient engine for some other stage of your journey, then ISP is a consideration. However, if you're simply not firing them because you think it's more efficient, then I would agree that you're better off leaving them behind. And if they are all the same engine type (or ISP), then firing them all together doesn't "cost" you any more.

[Woopert]

I agree with all of the above. When I came back from my KSP break, I came up with this prototype, cargo carrying lander that had two radial engines and two LV-909s. Probably shouldn't have done that (but I've learned a lot with my second stint at KSP).

[Bean]

If you post a pic of your vessel we can give a better answer, I often have engines on my ships that don't fire when they can (Clear path beneath them) because they are for another stage of the journey, mainly landing and taking off.